1. This invention relates to a display unit using liquid crystal.
2. Description of the Prior Art
Conventionally, liquid crystal displays of so-called super-twisted nematic (STN) type having liquid crystal molecules whose twist angle ranges from 180 to 270 deg. have been available as a unit that has a large display capacity with low-cost passive matrix structure. The STN liquid crystal displays are disadvantageous in that coloration is accompanied with use of the birefringent mode, thus research and development works aiming at those for displaying it in a monochrome pattern have been largely progressed recently. Out of which, what is most superior in display characteristic and can be expected to have a bright future is of the type which has two liquid crystal layers consisting of STN liquid crystal layer and optical compensation liquid crystal layer added thereto for the achromatic purpose. The liquid crystal displays structured as above are described in detail in, for example, Nikkei Microdevices, No. 10, pp. 84 through 88, October 1987. Also, for those having two-layer structure of TN liquid crystal with a twist angle of 90 deg., based on the same principle as above, to improve the display contrast, U.S. Pat. No. 4,443,065 by Funada et al is known. This two-layer type liquid crystal display unit is structured so that two liquid crystal layers whose retardations, i.e., the product (.DELTA.n.multidot.d) of the birefringence (.DELTA.n) of liquid crystal and the thickness (d) of the liquid crystal layer, and twist angles are approximately identical to each other, and whose directions to be twisted are reverse to each other may be arranged so as to make their molecular longitudinal axes approximately perpendicular to each other at the interface. The case when a voltage is not applied to the liquid crystal layer used for the display purpose, or it is applied at a value below the threshold will be considered here. Input light is linearly polarized through a polarizer and by successively passing through the liquid crystal layer used for the display purpose, the dependence of state of polarization on wave length is generated. However, the optical compensation liquid crystal layer compensates the dependence of state of polarization on wave length by being structured as shown above, thus the output light being returned to the original linear polarization independently of the wave length. As a result, if the plane of absorption of the electric vector of the polarizer disposed at the output side is arranged so as to be parallel with this, complete black display condition can be obtained. On the other hand, by appropriately selecting the values of the above-mentioned parameters, a pixel to which a selected voltage is applied can be displayed in the white condition. In this case, the state of polarization in the selected pixel is not perfectly linear, but slightly depends on the wave length. However, substantially satisfactory white display condition can be obtained in practice for the reasons that two liquid crystal layers, one is for the display use and the other for the optical compensation use, are directed so that the dependence on wavelength can be, although not perfectly, cancelled, and slight dependence of state of polarization on wave length or the existence of elliptically polarized light component cannot so largely effect on the white display condition as in the black display condition.
As described above, conventional liquid crystal displays are required to dispose one more liquid crystal layer than STN liquid crystal displays, which means that there have been arisen some problems such as a cost increase caused by manufacturing process or material cost and a increase in thickness or weight of the unit.